As an example of a conventional ultrasonic flowmeter using an ultrasonic flow velocity meter of this type, an ultrasonic flowmeter based on a propagation time difference is known. In such a current meter, an ultrasonic transducer is placed on the upstream side and downstream side of a conduit to be measured respectively and uses an impulse signal or step signal as a transmission signal, sends out ultrasonic waves from two ultrasonic transducers on the upstream side and downstream side alternately and measures the time difference in propagation of the received signal using a high-speed counter circuit.
However, such an ultrasonic flowmeter based on a propagation time difference is required to have a high signal-to-noise ratio to accurately determine the propagation time difference, and in the case of an impulse signal in particular, the reception time in an initial area of a waveform with small output needs to be measured precisely, which requires a high-voltage impulse signal or step signal. This requires a power supply circuit to generate a high voltage in the apparatus, which constitutes a great obstacle in downsizing of the apparatus and reduction of power consumption.
Furthermore, application of a high-voltage signal to the ultrasonic transducer limits its use in a place full of explosive gas.
Furthermore, an accurate measurement of an ultrasonic propagation time requires the use of a counter with a high count frequency, which increases power consumption and also constitutes a great obstacle against low power consumption in this sense.
Another known type is that a transmission ultrasonic transducer and a reception ultrasonic transducer are provided on the upstream side, another transmission ultrasonic transducer and another reception ultrasonic transducer are provided on the downstream side, continuous waves are sent from the transmission ultrasonic transducers on the upstream and downstream sides, which are received by the reception ultrasonic transducers on the downstream and upstream sides, phase differences between the received signals are measured and a flow velocity is calculated from those phase differences (e.g., Japanese Patent Laid-Open No. 7-167696). However, when used for such continuous waves, transmission ultrasonic transducers and reception ultrasonic transducers, at least two each, four in total are required, resulting in problems that the cost increases as the number of parts increases and the size of the apparatus increases and that consecutive transmissions and receptions will also increase power consumption.
The present invention has been implemented in view of such problems and it is an object of the present invention to provide a small, low cost and low power consumption ultrasonic flow velocity meter.